ADDER AdderLink Infinity 2112T User Manual page 45

Forwarding modes
In essence, the job of a layer 2 switch is to transfer as
fast as possible, data packets arriving at one port out to
another port as determined by the destination address.
This is known as data forwarding and most switches offer
a choice of methods to achieve this. Choosing the most
appropriate forwarding method can often have a sizeable
impact on the overall speed of switching:
• Store and forward is the original method and requires
the switch to save each entire data packet to buffer
memory, run an error check and then forward if no
error is found (or otherwise discard it).
• Cut-through was developed to address the latency
issues suffered by some store and forward switches.
The switch begins interpreting each data packet as it
arrives. Once the initial addressing information has been
read, the switch immediately begins forwarding the
data packet while the remainder is still arriving. Once
all of the packet has been received, an error check is
performed and, if necessary, the packet is tagged as
being in error. This checking 'on-the-fly' means that
cut-through switches cannot discard faulty packets
themselves. However, on receipt of the marked packet, a
host will carry out the discard process.
• Fragment-free is a hybrid of the above two methods.
It waits until the first 64 bits have been received before
beginning to forward each data packet. This way the
switch is more likely to locate and discard faulty packets
that are fragmented due to collisions with other data
packets.
• Adaptive switches automatically choose between the
above methods. Usually they start out as a cut-through
switches and change to store and forward or fragment-
free methods if large number of errors or collisions are
detected.
So which one to choose? The Cut-through method has the
least latency so is usually the best to use with AdderLink
Infinity units. However, if the network components and/
or cabling generate a lot of errors, the Store and forward
method should probably be used. On higher end store and
forward switches, latency is rarely an issue.
Layer 2 and Layer 3: The osI model
When discussing network switches, the terms Layer 2 and
Layer 3 are very often used. These refer to parts of the
Open System Interconnection (OSI) model, a standardized
way to categorize the necessary functions of any standard
network.
There are seven layers in the OSI model and these define
the steps needed to get the data created by you (imagine
that you are Layer 8) reliably down onto the transmission
medium (the cable, optical fiber, radio wave, etc.) that
carries the data to another user; to complete the picture,
consider the transmission medium is Layer 0. In general,
think of the functions carried out by the layers at the top
as being complex, becoming less complex as you go lower
down.
As your data travel down from you towards the
transmission medium (the cable), they are successively
encapsulated at each layer within a new wrapper (along
with a few instructions), ready for transport. Once
transmission has been made to the intended destination,
the reverse occurs: Each wrapper is stripped away and the
instructions examined until finally only the original data are
left.
So why are Layer 2 and Layer 3 of particular importance
when discussing AdderLink Infinity? Because the successful
transmission of data relies upon fast and reliable passage
through network switches – and most of these operate at
either Layer 2 or Layer 3.
The job of any network switch is to receive each incoming
network packet, strip away only the first few wrappers to
discover the intended destination then rewrap the packet
and send it in the correct direction.
In simplified terms, the wrapper that is added at Layer 2
(by the sending system) includes the physical address of
the intended recipient system, i.e. the unique MAC address
(for example, 09:f8:33:d7:66:12) that is assigned to every
networking device at manufacture. Deciphering recipients
at this level is more straightforward than at Layer 3, where
the address of the recipient is represented by a logical IP
address (e.g. 192.168.0.10) and requires greater knowledge
of the surrounding network structure. Due to their more
complex circuitry, Layer 3 switches are more expensive
than Layer 2 switches of a similar build quality and are
used more sparingly within installations.
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